1. Field of the Invention
The present invention relates to a structure of a power module, and more particularly to a structure of a power module comprising an insulating substrate having a power semiconductor device mounted thereon and a control substrate on which a control IC for controlling the power semiconductor device is mounted.
2. Description of the Background Art
FIGS. 5 and 6 are perspective and sectional views showing a structure of a conventional power module. As shown in FIG. 6, the conventional power module comprises an insulating substrate 105 and a control substrate 113 on which circuit patterns (not shown) are formed respectively, interconnection leads 110 and 120, and an insulating case 107. A power semiconductor device 109 is mounted on the insulating substrate 105 through a solder 108. The insulating substrate 105 is provided in contact with a metallic base plate 102 through a solder 104.
A control IC 115 for controlling the power semiconductor device 109 is mounted on the control substrate 113 through a solder 114. One of ends of the interconnection lead 110 is electrically connected to the insulating substrate 105 or the power semiconductor device 109 through a metal wire 111. The other end of the interconnection lead 110 is electrically connected to an electrode 122 provided on the control substrate 113. The electrode 122 is electrically connected to the control IC 115. One of ends of the interconnection lead 120 is electrically connected to the power semiconductor device 109 through a metal wire 121. The other end of the interconnection lead 120 is electrically connected to an electrode 123 provided on the insulating case 107.
The insulating substrate 105, the control substrate 113 and the interconnection leads 110 and 120 are provided in the insulating case 107. An internal space of the insulating case 107 provided below the control substrate 113 is filled with a silicone gel 112. Moreover, a cover 117 is fixed to a top of the insulating case 107. The control IC 115 is electrically connected to a control connector 119 provided on the cover 117 through an external connecting terminal 118 soldered onto the control substrate 113.
The base plate 102 is fixed to a heat sink 101 with a bolt 103. A thermal conduction grease 116 is coated comparatively thickly (approximately several hundreds .mu.ms) between the base plate 102 and the heat sink 101.
According to such a conventional power module, however, an electromagnetic shield is not fully formed for the insulating substrate 105 and the control substrate 113. Therefore, there has been a problem in that the power module is easily influenced by external noises made by a driving motor or the like and reliability is deteriorated.
Moreover, the base plate 102 and the heat sink 101 are fixed to each other with the bolt 103. For this reason, it is necessary to provide a space for forming a bolt hole for the bolt 103 in an upper surface of the heat sink 101. Consequently, there has been a problem in that the size of the heat sink 101 is increased.
Furthermore, a warp is easily generated on the insulating case 107 and the base plate 102 by heating for thermosetting the silicone gel 112 and heating for bonding the insulating substrate 105 to the base plate 102. Consequently, the thickness of the thermal conduction grease 116 cannot be reduced so that a thermal resistance is increased and heat-radiation effects produced by the heat sink 101 are reduced.